Sebastien LEBONNOIS

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IPSL Venus GCM

Model and main results

In the current version of the Venus GCM, based on the LMDZ5 dynamical core, we use a horizontal resolution of 96 longitudes by 96 latitudes (3.75x1.875 degrees), on 50 levels from surface to roughly 95 km altitude.
The orbit of Venus is taken as circular, and we neglect the inclination of
Venus' rotation axis. We take into account the topography.
The model is described in Lebonnois et al. (2010, 2016). Latest results are in Garate-Lopez and Lebonnois (2018).

Variable Cp:
The standard version of the LMDZ dynamical core uses a single value for the
specific heat Cp, but Cp varies in the atmosphere of Venus from
738 J/kg/K at 100 km altitude to 1181 J/kg/K near the surface.
This variation of Cp with temperature needs to be taken into account,
in order to get realistic adiabatic lapse rates in the whole atmosphere.
Using an analytical description of Cp(T), I have taken this into account in the dynamical core with a new expression of the potential temperature.

Radiative transfer:
For the moment, we use tabulated solar fluxes from Haus et al. (2014).
However in the infrared, we use Net Exchange Rates matrices (Eymet et al., 2009) which allow a consistent computation of the temperature field.
This is different from previous GCMs of Venus' atmosphere, which were based
on simplified forcing of the temperature structure, and it has a crucial impact on the meridional circulation.

Mean circulation:
Superrotation is obtained at the cloud level, above roughly 40~km altitude, but
below this altitude, the zonal wind does not increase to the observed values.
This discrepancy is a major pending question, though our studies emphasize the role of different wave activities on the angular momentum redistribution, in particular near the cloud-base and in the deep atmosphere.
The meridional circulation consists of equator-to-pole cells, with the dominant
one located within the cloud layers.
The modeled temperature structure is globally quite consistent with
observations. A peculiar feature of the polar areas, the cold collar, is now obtained in the simulations. We showed the impact on the formation of this structure of the latitudinal variations of the clouds. This coupling between cloud structure, radiation field and circulation needs to be fully taken into account in future improvements of the GCM.
A convective layer is found between the base of
the clouds (around 47~km), heated from the deep atmosphere below and the
middle of the clouds (55-60~km altitude), region that is able to cool
directly to space; this is consistent with observation of the stability
structure above 40~km.

Additional processes:

The implementation and study of the photochemistry was initiated with the PhD of Aurelien Stolzenbach at LATMOS under the direction of Franck Lefevre.

The microphysical module has been developed by Sabrina Guilbon during her PhD at LATMOS under the direction of Anni Määttänen.

Difficulties

Our GCM simulations (as well as those of other teams) show that the superrotation of Venus atmosphere is the result of a subtil equilibrium.
It involves balance in the exchanges of angular momentum between surface and atmosphere, and balance in the angular momentum transport between the mean meridional circulation and the planetary waves, thermal tides, and gravity waves.
Modeling this balance is sensitive to the dynamical core details, to the
boundary conditions and possibly also to initial conditions.

The sensitivity to the dynamical core is illustrated by the many differences
obtained in the recent modeling of Venus circulation, but has been more formally demonstrated in a comparative study between Venus GCMs under identical physical
forcings conducted in a working group at ISSI, Bern, Switzerland (see Lebonnois et al. 2013).
These studies revealed that various dynamical core which would give very similar
results in Earth or Mars conditions, can predict very different circulation
patterns in Venus-like conditions.
The wide dispersion of the modeled wind fields in these studies has to be related to the various dynamical core implementations,
through angular momentum conservation and/or horizontal dissipation processes.
Since the planetary waves play a crucial role in the angular momentum balance,
their representation in the spatial and temporal discretization mechanisms
of the dynamical cores may also be part of this sensitivity.

Since they control the exchanges of angular momentum between the atmosphere and
the surface, the lower boundary conditions used in the model also have a strong
influence on the resulting wind field. The planetary boundary layer (PBL) scheme
has an impact on the surface friction, but also on the temperature structure of
the deepest atmospheric layer, therefore affecting the circulation close to the
surface. The presence of topography introduces the mountain torque in the
surface-atmosphere exchanges. This torque results from different surface
pressures on the eastern and western (for its component affecting the zonal
wind) sides of topographical features. It is a major component of these angular
momentum exchanges, as shown e.g. in Lebonnois et al. (2010).

The sensitivity to initial conditions with such models is also a problem.
Kido and Wakata (2008) first obtained different strengths for
the zonal wind field when starting from rest or from pre-existing superrotation. Similar results were obtained during the ISSI inter-comparison study
(Lebonnois et al. 2013), and also in recent simulations with the LMD Venus GCM.

Future studies

Among the studies we want to conduct in the near future:

Investigate the sub-grid processes such as gravity waves with a mesoscale model, to characterize these processes and improve their parameterizations in the GCM. This work was initiated with the PhD of Maxence Lefèvre (co-supervised by my colleague Aymeric Spiga and myself).

Compute the solar fluxes using a Monte-Carlo 3D code developed by Vincent Eymet to get a consistent computation of all the radiative transfer.

Increase the resolution and investigate its impact on waves, circulation and angular momentum and heat budgets.

Implement the new icosaedral dynamical core DYNAMICO to investigate the impact of this new core on the angular momentum budget and on the polar circulation.